A water-containing combustible solid such as sludge has a high water content from 70 to 80% by weight of the sludge after simple dewatering by belting press, roll press or the like. Accordingly, in order to incinerate the sludge in a fluidized incinerator, drying by a hot air drying machine is carried out, and a large amount of energy is consumed for the drying. However, since sludge still contains water in an amount from 30 to 50% by weight of the sludge after being dried, complete combustion of the sludge by a fluidized incinerator is difficult. Accordingly, the unburned combustibles remaining in ash after combustion are burned by petroleum or natural gas under the present conditions. In addition, there are the disadvantages of the fluidized incinerator requiring a huge equipment cost, and requiring a high running and the like. Wood waste discharged from a lumber mill or the like, paper sludge, or the like is similarly difficult to combust completely by a fluidized incinerator, and is incinerated by the similar method.
Accordingly, a method is desired, which is capable of not only easily removing water from a water-containing combustible solid, but also recovering the water-containing combustible solid in a form suitable for incineration.